Standard electric meter provided with portable stroboscopic device



Dec. 5, 1950 G. E. MAILLAT STANDARD ELECTRIC METER PROVIDED WITH PORTABLE STROBOSCOPIC DEVICE 5 Sheets-Sheet 1 Filed April 9, 1946 Dec. 5, 1950 G. E. MAlLLAT 2,532,

STANDARD ELECTRIC METER PROVIDED WITH PORTABLE STROBOSCOPIC DEVICE Filed April 9, 1946 3 Sheets-Sheet 2 1) Dec. 5, 1950 G. E. MAILLAT 2,532,416

. STANDARD ELECTRIC METER PROVIDED WITH PORTABLE STROBOSCOPIC DEVICE Filed April 9, 1946 3 Sheets-Sheet 3 117/1; 6277601 6' Z. Kai/66475 Patented Dec. 5, 1950 STANDARD ELECTRIC METER PROVIDED WITH PORTABLE STROBOSCOPIC DEVICE Gustave Ernest Maillat, Seine-et-Oise, France, assignor to Electricite de France, Paris, France, a body corporate of France Application April 9, 1946, Serial No. 660,593 In France March 3, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires March 3, 1964 2 Claims.

The invention exposed hereinafter has for its object an improvement relating to standard electric meters provided with a stroboscopic device in a portable case, in particular to the meters designed previously by the same inventor for a quick verification on spot of relative errors in electric induction meters belonging to low tension systems.

This improvement has for its main objective the rendering of the portable apparatus still lighter and more convenient in use, thanks to a number of modifications.

-The chief modification relates to the stroboscopic device.

The known principle of the stroboscopic verification method consists in placing the standard meter in the same electric conditions as the subscribers meter and in confirming by use of the stroboscopic method that the guidemarks of their respective discs rotate at the same speed. For this purpose, the disc of the subscribers meter is provided on its lower face with a circumferential graduation; the disc of the standard meter, furnished on its edge with a comb, or with teeth, having an equal number of steps with the subscriber's graduation, is interposed in a track of a luminous beam falling on the said lower face of subscribers meter disc; the standard disc gives thus rise to luminous modulations which cause 'an apparent immobility due to stroboscopic efiect, at the moment when the modulation frequency is equal to the displacement rate of the graduation marks of the subscribers meter disc. The observation of this graduation is effected through an aperture in the meter case which delimits only a small area of the graduation, for example 1 cm. approximately, the number of graduation lines per centimeter is high, e. g. 30 to the cm.

In the earlier instruments, mentioned above, the author has foreseen an illumination of the subscribers disc by a limited number of light beams produced by a small lamp and a stationary comb teeth, placed immediately below the edge of the standard meter disc, this edge being formed by a photographic film having a circular graduation identical with that of the comb. This film, when rotating, intercepted or passed the group of luminous beams, which were received then by an optical glass instrument pro ducing on the lower face of the subscribers meter disc a difiused illumination, modulated by the rate of displacement of the standard meter disc graduation. When the two rotation speeds were equal, the subscribers meter disc appeared to be stationary.

This stroboscopic device exhibited several drawbacks, in particular the following:

1. The optical glass device needed was relatively heavy and expensive.

2. To obtain an illumination on the subscriber's meter disc formed as one diffused area and not as separate luminous areas corresponding to the light beams, it was necessary to have the distance between the subscribers meter disc and the stroboscopic devices of the standard meter of a definite determined value and what follows the case of the standard meter fitted at a definite determined position in respect to the meter to be verified. A special device for con meeting and branching in of the standard meter case circuits was, therefore, needed.

The improvement in the stroboscopic device according to the present invention overcomes these drawbacks.

It is characterized by the fact that the stroboscopic device is reduced toa point light source projecting directly, without the use of optical glasses, a shade of the standard graduation thrown clearly on the subscribers graduation, these two graduations rotating either in the same direction or in the opposite direction.

The shade of the standard comb is projected on the disc graduation of the meter to be verified with a magnification in the proportion of 3:1 or of 4:1, in a way that 3 or 4 graduation marks of the disc to be verified appear between the shades of two successive marks of the, standard graduation. As result of this magnification, the circumferential speed of the shade thus projected is about three or four times higher than the circumferential speed of the disc of the meter to be verified, the successive sweepings in the vision field of the large black marks constituted by the shades of the marks of the standard graduation, produce an effect equivalent to that of periodic illumination interruptions used for stroboscopic effects.

The observation shows as a matter of fact that in these conditions, if the displacement rates of the two graduation marks are equal, the graduation of the subscriber appears to be stationary, whereas if they' differ from one another, this graduation seems to rotate in one or in the other direction according to the sign of the difference.

The chief advantages of this device represent the elimination of-optical glasses and of the stationary comb, resulting in weight decrease and economy, as well as in suppressing the necessity of placing the standard meter in an exact position in respect to the subscribers meter; in these conditions the standard apparatus may be held in hand as is held a pointing photographic camera.

The present invention comprises, moreover, other arrangements, which tend to render the device more readily portable and more conent 11 se; these arrangements are described no longer designed to be fitted on a control plate which ensures at the same time its branching to the line, is provided with connections,.which.

can be made by means of a. special contact having the chief advantage otpermittingto-place the standard resistances outside the standard meter box and thus allowing ct -further; decrease.

in weight.

For examples sake an apparatus according to the invention was described hereinafter and.

shown on the accompanying drawings; it was designed to enable a quick verification of the monophase induction meters as used for low tension. In the case taken as example, the meter to be verified is identical to the standard meter contained in the'stroboscopic apparatus.

Fig. 1 shows in perspective view the-device according to the invention, connected to the special contact, as'mentioned above, andplaced in the operating'position in front 10f the =meter to be verified.

Fig. 2'gives a diagrammatic view of the connections and the 'inner structure of the device according to the-invention.

Fig. 3 shows in axial longitudinal section the projection lamp and the box where-it is lodged.

Fig. 4 shows the window provided-with an intion is shown in the .box I, in which only the 1 disc 2 of the standardmeter and a'small electric lamp 3 fittedina diaphragm case t are shown.

The disc 2 carries 'on its rim a circular photographic film 5 on which are impressed uniformly distributed black radial bands, which areof the same width as theintermediate-translucide portions. (In the figure the graduation'proportion has been considerablyexaggerated.)

The above mentioned-luminous source 3 placed tilted below the disc 2, illuminates. a small film area 5, theshade of which is projected on to the lower edge side of the graduated disc fi-of the meter which has to be verified, through a window 8 arranged in thefront wall of the box. In the present case the number of the equidistant radial marks'of the disc graduation is equal to the number of blacks: in: the film 5.

The luminous beams which crossedthe window ingoing in. are difiused by the vdiscfi :and are partially reflected through this. window. They arrive at the operators eye after being refiectedin a concave mirror 9 fixed on the box 1..

Inacase where'the device according to the invention is placed higher than shown in Fig. 1, atuthegheight of'theoperators eye, a small'flat mirron (not. shown inydrawing) will be used to send the beams back to the eye; it willbe placed suitably above the ,said:,concave .mirror, and forming aperiscope withsit.

The device is connected to a special contact It (fixed above *the.subscribers'mainpswitch ll) through a three conductor cord I 2 'anda-multiple plug l3, inserted in the socket H). The control resistance 16 is branched to the same socket. It allows a very short'of needed magnitude output just necessary for the Verification to pass through the meter, and comprises for this purpose a small high resistance lamp [5, connected in parallel with it. The operation of the switch 16 allows the lamp [5 to remain alone in circuit.

According to Fig. 2 showing diagrammatically the connections corresponding to the device in Fig. 1, the meter to be verified ll comprising an intensity winding l8 and a tension winding I9,

L is branched to the line 29 by two inlet terminals.

From this meter I! on starts the main subscribers line 2|, supplying the receivers. 22 through the main'switch 23.

The above mentioned special current contact 24 1s connected in parallel to the main subscribers line, above the main-switch 23.

It comprises chiefly three terminals. 25, 26, 2! forbranching in of the stroboscopicverification device 23 and two terminals 29, 30 for branch.- ing in of the removable resistance "elements.3l (shown by I4 in Fig. 1).

The device according to the invention consists, as was explained above, of a stroboscopic observation device and a standard meter. In. the Fig. 2 are represented the intensity 32, and'lten sion windings 33 of this meter. The latter Windings is fed through a potentiometer 34 connected to the terminals 26 and 2'! of the contact 24. One end of this potentiometer is shunted by a rh'eo stat 35, the steps-ofwhich are calibratedas will be explainedbelow. The other end of the potentiometer presents two adjustment terminals 3'6 and 3'! and a movable arm 38. By placing the arm 38 on the'terminalstfi, a: current higher by 3% than the normal current, corresponding to the pressure appliedtothecir cuit terminals, passes through the tension winding'33.- By'placing the arm on the terminal 31;

. lamp- 39 (shown .by 3 in Fig. 1) is supplied by a small transformer 51],"the primary of which is formed by the tension winding 33 of the standardmeter.

The removable resistance element 3! comprises two resistances in parallel. One, very higlufll, may be constituted by a small lamp (shown by 25 in Fig. 1); it will absorb a negligible current. The other resistance 42 is chosen bythe verify-- ing operator to obtain suitable working conditions for the verification of the two meters. The switch 43 (shown by 16 in Fig. 1) allows a short working time of a few seconds only of the re-. sistance 3| (the verification time). This very light resistance of the electric iron type, being enclosed in a tight box cannot be, in effect, held longer than a very short time in circuit.

Inflow-tension. meters, the .error of recorded difierent loadingconditions ought .not over top the given maxima and minima (usually 13%).

The verification will then'consist in giving to the standard. metenwithhelp of the switch 38, and for each of the load conditions required for the verification, a relative error equal to +3% or 3% respectively, and in comparing, in each case, the working of the two meters. This comparison will be efiected by means of the disc veloci-ty'c'omparison according to the'stroboscopic method described above. In case when the meterunder examination works withinthe admissible error limits, it ought then at any load conditions rotate slower than the standard meter, when the arm 38 is on the step 36 (relative velocity increase +3%) and rotate faster than the latter when the arm is on the step 31 (relative velocity decrease -3%). After the verifying attendant has suitably adjusted the rheostat 35 and inserted the resistance 42 to obtain the required working conditions for the verification, he will operate the arm 38 as was said above. For each of the arm positions, 36 and 31, he will observe the relative movement of the graduation of the disc 6 (Fig. 1) belonging to the meter to be verified.

If the meter to be verified is well adjusted, the graduation of the disc of the meter under test will appear to move, during the first test, in the opposite direction to its real motion. On the contrary, during the second test, it will be seen to move in direction of its real movement. To carry out a similar verification at very light load, it suffices to open the contacts 23 and 43. The very high resistance 41 is then-alone in circuit, connected to the terminals of the meter.

For verification by use of the standard apparatus 28 of induction meters whose constants per revolution (the amount of energy recorded per revolution) are different from those of the standard meter, it is, of course, necessary to have the number of equidistant radial marks traced on the lower face of the respective discs, in contradiction to what was foreseen hereinbefore, different from the number of teeth in the standard meter film. Give, for example, a consumers meter of 5 amperes under 110 volts to verify, the

constant per revolution of which is 0.8 watt-hour (that is to say the meter records 0.8 watt-hour per revolution) with help of a standard meter of 5 amperes under 110 volts, whose constant per revolution is 1 watt-hour and whose disc has 520 circumferential teeth.

If the meter to be verified is exactly adjusted for the predetermined working conditions, the graduation of this disc observed by means of the above described apparatus will appear to be stationary if the numbers of marks on the two discs are proportional to their constants per revolution, it means if, in the first case, the number of marks of the disc under test will be equal to 520 0.8=416. As the figures thus obtained do not correspond to a convenient disc graduation, a graduation of 400 marks will be used with a fol lowing reduction of standard meter disc speed obtained by the operation of the rheostat 35 for the verification of the working of this type of meter. This rheostat is calibrated corresponding to the constants per revolution of the meter that will undergo the verification of the attendant.

As shown in Fig. 3, the lamp 44, of the type used in the electro-medical apparatus, is situated in a cylindrical box 45, the top of which being of considerable thickness is holed at its center bya circular aperture 46 having few tenths of millimeter in diameter. This arrangement gives the required point source of light. The image of the graduation 5 projected by means of this pointlight is perfectly clear and sharp.

Fig. 4 shows the shape of the window 41, provided with a blackened passage 48, which allows to observe the graduation illuminated by means of the lamp 44 even when operating in full sun light.

In Fig. 5 which shows the embodiment of the contact as represented diagrammatically in Fig. 2, the consumers meter under test, connected to 6 the network It is denoted by 50, the base of the contact by 5!, the corresponding multiple plug by 52, the standard meter provided with the stroboscopic device by 53, the removable resistance by 54, the lamp support by 55 and a connecting piece by 56. To show the inner connections of these members, they are represented as if transparent.

The base 5| exhibits On its front face three main sockets 51, 58, 59 constituting an extension of threaded pins serving, on the one hand, to fix the base to the supporting board, on the other hand to connect the sockets to the inlet leads 63, 64, 65 of the meter 50, the multiple plug 52 carries three plugs 66,61, 68, fitting in the sockets 51, 5B, 59 respectively. These plugs 66, 61, 68 are connected to the leads 69, 10, H respectively, which leads form a flexible cord terminating at the stroboscopic verifying apparatus 53, the inlet terminals of which are shown by 12, 13, 14. This apparatus comprises a standard meter whose intensity winding is denoted by 15 and the tension winding by 16.

The three main sockets 51, 58 and 59 are connected to the auxiliary sockets H, 18, #9 respectively and are situated on the upper face of the base 5|. Into the sockets TI and 18 can be fitted the two plugs of the connecting piece 56, whereas into the sockets 18 and 19 can be inserted plugs and BI of the resistance element. These plugs 86 and BI are connected through the leads 82 and 83 to the sockets 84 and 85 respectively, these sockets being situated in the upper part of the said element 54. The. lower part of this element contains a standard resistance, diagrammatically represented at 86, of the electric iron type and connected in parallel to the leads 82 and 83. A double-pole interrupter 8! branches in or disconnects the resistance 86 circuit.

For test purposes use is made of a set of elements 54, which differ between themselves only by magnitude of their resistances 86. The resistances 86 range, for example, from 50 watts up to 500 watts, or even 1000 watts for some heavy load meters.

Into the sockets 84 and. 85 of the resistance box 54 enter the plugs of the lamp support 55, the filament 96 of this lamp being of very high resistance.

The distance between the plugs 88 and 89 is equal to that between the plugs 85 and 8!, the support lamp 55 may thus be connected either to the resistance element 54, or to the base 5|.

The subscribers meter 50 under test comprises. an intensity circuit 9| and a tension circuit 92. Its inlet terminals are denoted by 93, 34, its outlet terminals by and 96, the phase conductor by 63, the neutral conductor by 57, the main subscribers interrupter by 98 and consumers circuit by 99.

When not under test, the connecting piece 55 is plugged into the sockets H and 18 of the base 5|, it ensures the continuity of the conductor 63 supplying the consumers installation. The verification of the meter 59 exhibits a double character'. Firstly, it' aims to ensure that the maximum error at light, medium and heavy load does not exceed, in plus or in minus, a predetermined percentage; for this verification sake the stroboscopic standard meter is adjusted. Secondly it allows to ensure that the meter works for a very small intensity of the order of 1/50 of the normal current. To set the standard meter in working conditions, themultiple plug 52 is put in first of all, the connecting-piece 56 is then taken out.

Thus 2 the branching: in. is effected withoutthe subscribersreceiving: circuit being cut. The

verification-of the percentage error magnitude otstherconsumers meter begins.

Threercasesmay occur:

First case-The installation is only under pres sure, the load'being zero.

Second caseConsumer charge.

Tliird' case-Consumer absorbs a heavy load, sufiicientto carry out the corresponding test.

Torcarry out the test in 'the-firstcase (installation' underpr-essure only) for. the heavy load, arsuitably'chosenresistance'element 54 is inserted absorbs a medium inzthesbase 5], the switch, of which 31 is open.

standard meter, exits out of this winding through the terminal I3, the conductor 'lfiland the socket 58.: It:passes-then' entirely through the socket i8 and the conductor 32 and" there into tworesistances; 86 and 99 mounted in parallel.

thefsocket 59, conductor-fiend the neutral conductori l towards the meter 59.. The resistance ofthe lamp 95 being very high, it has no practical infiuencein the adjustment of the loads of the two meters. toflow when the interrupter is open, the lamp preventsth-e passage of any violent extra currents'oi opening and of closing in the meters, when they are branched in. or disconnected by the interrupter 81, mounted in the element 54.

Tocarry out the medium and light load .tests,

use is'made of other elements 'such as 54, by inserting each time the lamp-support 55. To carry out a very light load test (starting of the meter) it is sufficient to open the interrupter 81.

In the second case (consumers medium load), the heavy load test is carried out by insertion of an element 54 whose resistance 86, put in parallel with the resistances 99, gives the over-all load necessary for efiecting of the verification. The

medium and light load tests are carried out together with reception apparatus es of the consumer. The very light load test is effected by opening of the subscribers interrupter 9S and with help of the lamp-support 55, plugged in directly in the base 5L Inthe third case (consumers heavy load) the heavy load test is carried out directly.

The lamp-support 55 being attached directly to the socket base 5!, the very light load test will be carried out, moreover, by opening the main interrupter 53.

When the. verification observations are finished, the connecting piece 55 is replaced and the multipleplug 52 taken out.

Fig. Sshows, in anarrangement similar to that ofctheFige 5, another form of the socket base situated in the meters terminal box. This base is used together with a triple plug, identical to the Plug-.551.- in Fig. 5;

It. flows through the'conductor 83, the'socket '19, 1

By allowing a very weak current This Fig. 6 shows a" meter IM to be tested; its intensity winding" Isl, its tension windingIM! The outlet ofthe winding Iii'I is'connected to the phase conductor I I2 by means of sockets IUSand ltd-which may beconnected together or isolated from one another with help of the removable connecting-piece H3. The outlet of the tension winding ductor HI'bymeans -of the terminal'mll. The main interrupter or the subscriber is shown at I14; its resistance elements a-t II5I- The meters I53 terminal box is divided into two compartments separated from each other'by a partition. The compartments I69 on the left; comprising the inlet terminals is covered by a sealed top'for preventing any theft'of-power by tapping in front the meter; the other compartment' I it is provided with a hinged cover-(not shown).

The compartment IIG comprises, moreover,

the sockets l53'and its, three other sockets I55," I55 and IE)? which are connected to thesookets- I633, E 34 and 'l 58 respectively, are the main sockets of the socket base and servefor branching in-of the measurement apparatus; thesockets ISS and it i in which can be inserted, as was-mentioned already, a-connecting piece I I3, are auxiliary base sockets.

Tobe able to branch in the measurement apparatus and the circuits indispensable for its op eration, use is made of a plug-socket member HS identical with that 5! in Fig. 5, which is interposed between the mainsockets and 'the multiple plug II'I. This plug-socket block is shown in Fig. 6 under tWo different forms:

(a) Viewfrom below (upper figure), the block contact being tilted;

(6) Plan view (lower figure); the block contact is shown as transparent.

Each of the three plugs H8, H9, I253 ofthis plug-socket block (upper fig.) forms'a common member with the corresponding socket of the same-denoting number (lower fig).

The sockets I2I and I22 connected to the sockets H9 and I20 respectively are also situated in the block and are used for branching in of control elements such as I23. 5 and 6, the following relationship between the sockets of the base 5! (Fig. 5) and those of the block II 6 (Fig. 6) can be noticed.

The-socket H8 in Fig. socket 5? in Fig. 5.

The socket H9 in Fig. 6 socket 58 in Fig. 5.

The socket I26 'in- Fig. 6'correspondsto' the socket 59 in Fig. 5.

The

corresponds to the the sockets I26, H9, H8 respectively. A resistance element I23 identical with the element 54 in Fig. 5 is plugged in on the side of the triple plugmember H5. of the meter does not allow to locate there a socket base, whose 'side'would be accessible for introduction of a'resistance element-12S; for this reason a member H5 providedwith main and auxiliary sockets and fitting'in the'sockets I95, I06, Ill? of the meter terminal box was designed in a manner to remain outside the latter and to have its auxiliary side sockets I2! 122 readily accessible.

the sockets 12 I, I22, in such a way'that the lampsupport (not. shown in. Fig. 6) (identical to the lamp-support 55 in Fig.5)- mayLbeipluggedieither I92 is connected to the neutral 0011- By comparing Figs:

6 corresponds to the plugs I24, I25, 25 of the multiple plug I I! of the standard stroboscopic meter enter into The depth of the terminal box- The. distance between the sockets- I2? and I28 of this element is equal to that ofin the element I23, or in the auxiliary member H6. The verification tests of the meter are executed as was explained already. For examples sake, the case of a monophase meter was taken in the preceding description, though the invention is not limited to this simple case.

As a summary indication, it will be shown how operates a socket base for a low-tension threephase meter connected to the network by three non-balanced conductors. Such a meter is provided with two intensity and two tension Windings. Its verification by an identical type meter will necessitate the provision in the socket member according to the invention disconnections of the two phase conductors and a connection to the third conductor; each disconnection will com-' prise four sockets: two for the connecting-piece, two for the branching in of the apparatus, the third conductor connection will comprise one socket.

The socket member, making object of the invention, may, for example, be designed for verification tests of all meters belonging to the multiple phase low-tension networks, irrespectively of the adopted distribution system and the number of phases.

In the case where the device is not only required for vertificatio-n purposes but for the calibration of the meters, this device will be attached on the cover case of the meter to be calibrated by any suitable connecting means, in the same position as occupied during the test or when held in hands, this arrangement permitting the operator to carry out the adjustment of the meter to be calibrated when working, without the need to stop or disconnect the stroboscopic device. In which case, the necessary connections between the stroboscopic device and the meter may be eifected by means of one cord only, by inserting directly in the sockets of the terminal box (Fig. 6) plugs integral with the box of the stroboscopic device.

As shown in Fig. 7, the different calibration loads necessary for the calibration are obtained by means of a small transformer rheostat. In this figure, the meter to be calibrated is denoted by 200, its intensity winding by 26!, its tension windings 20- 2, 204 and 205 denote the corresponding windings of the standard stroboscopic meter 203. A transformer 208 fitted on its primary with a rheostat 266 is supplied from any suitable point of the installation, for example through a switch 201 enabling to disconnect the rheostat. The secondary of the transformer is branched, for example, by means of simple tappings, between the terminals of the intensity winding 26! of the meter 206. Under these conditions the current intensity in the intensity windings of the two meters is determined by the difierence of the intensity absorbed by the consumer and the intensity resulting from the induced tension in the transformers secondary 208, which is adjusted by the rheostat 206.

A differential arrangement is thus obtained permitting to place the two meters in required conditions for calibration at any input absorbed by the subscriber, and even to carry out a reversed test to examine the working condition of the mechanical members of the meter.

The transformer rheostat being of low power, of the order of 4 to 5 volt-amperes and having very small dimensions, it may be fixed inside or on the outside of the stroboscopic device box.

What I claim is:

1. A stroboscopic device for checking and adjusting, by means of a standard meter, a service meter having a disk, radial graduation marks on the edge of the lower face of said disk, a window on the front of a casing containing said service meter for observing said graduation marks and means for readily connecting the standard meter to the meter to be checked, wherein the said standard meter comprises a casing, a disk rotating at the same angular speed as the disk of the service meter, alternately translucid and opaque radial graduation marks of equal width on the periphery of said standard meter disk, a punctual source of light positioned under the last said disk for throwing over the lower face of the disk of the meter to be checked enlarged images of said opaque and translucid marks.

2. A stroboscopic device for checking and adjusting, by means of a standard meter, a service meter inserted between a supply network and a consuming network and having a disk, radial graduation. marks on the edge of the lower face of said disk, a window on the front of a casing containing said service meter for observing said graduation marks, a first compartment in the lower part of said casing comprising the connections of a current circuit and a potential circuit of the service meter with the supply network, a second compartment in the lower part of said casing comprising two sockets connected respectively to the free ends of said circuits, two sockets connected respectively to the terminals of the consuming network, a first bridge-connection between the socket connected tothe free end of the potential circuit and the socket connected to one terminal of the consuming circuit, a second bridge connection between the socket connected to the free end of the current circuit and the socket connected to the other terminal of the con.- suming circuit, a last socket connected to said second bridge, wherein the said standard meter comprises a casing, a disk rotating at the same angular speed as the disk of the service meter,

' alternately translucid and opaque radial graduation marks of equal width on the periphery of said standard meter disk, a punctual source of light positioned under the last said disk for throwing over the lower face of the disk of the meter to be checked enlarged images of said opaque and translucid marks, a 3-wire lead connected to the current and potential circuits of the standard meter, and a plug at the free end of said lead having three pins cooperating respectively with the last socket onv the second of said bridge and with the sockets of the said first bridge.

MAILLAT, ERNEST GUSTAVE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 905,966 Wilkins Dec. 8, 1908 1,427,776 Blathy Sept. 5, 1922 1,864,771 Sparkes June 28, 1932 2,123,115 Maillat July 5, 1938 2,267,282 Larson Dec. 23, 1941 

